科普-陨石简论.pdf
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1、1Meteorites: an overview陨石简论陨石简论Edward R. D. Scott(本文译自Elements 2011年7卷1期47-48页)Meteorites come from numerous parent bodies with a wide variety of geological histories. A few(0.5%) come from Mars or the Moon; the rest are impact debris from collisions betweenasteroids orbiting between Mars and Jupit
2、er. Unlike terrestrial, Martian, and lunar rocks, theasteroidal meteorites contain minerals that formed before the Sun and the Solar System, during thegrowth of planetesimals and planets from the disk of dust and gas around the Sun (“the solarnebula”), and during the first half-billion years of Sola
3、r System evolution.陨石来自各种经历过不同地质作用的母体星体。 除了少量陨石 (除了少量陨石 (0.5%) 来自火星或月球外,其它的陨石都来自火星和木星之间的小行星带。) 来自火星或月球外,其它的陨石都来自火星和木星之间的小行星带。与地球、火星和月球岩石不同的是,小行星陨石包含了太阳和太阳系形成前, 行星从太阳星云形成过程中, 以及太阳系演化过程最初5 亿年的矿物。Meteorites from unmelted asteroids are called chondrites; they are cosmic sediments composed ofparticles th
4、at were present in the solar nebula (Fig. 1). The most abundant particles aremillimeter-sized objects called chondrules, which are solidified droplets of silicate magma. Thechondrules, associated grains of FeNimetal, and a few volume percent or less ofcalciumaluminum-rich inclusions (CAIs), which we
5、re the first solids to form in the solar nebula,are embedded in a fine-grained matrix. The matrix initially contained a mixture of chondrulefragments, presolar and solar nebular grains, and organic materials. However, most chondriteswere aqueously altered or thermally metamorphosed so that only a ha
6、ndful of chondrites havepreserved presolar grains and pristine chondrules.球粒陨石球粒陨石来自未经历熔融分异的小行星, 它们是由来自太阳星云的尘埃经宇宙沉积形成 (图1)。最常见的尘埃就是毫米级的球粒,它由硅酸盐岩浆凝结形成。球粒,Fe-Ni金属以及少量富钙铝包体(CAIs,太阳星云中最先凝固的物质),镶嵌在细粒结晶的基质中。基质包含球粒碎片,前太阳和太阳星云物质,以及有机物质。然而,大部分球粒陨石后期会经历水蚀变或热变质,所以,只有少部分球粒陨石能保留前太阳物质和原始球粒。The three major classes
7、 of chondritescarbonaceous, ordinary, and enstatiteare divided into 13major groups. Ordinary chondrites, which make up 85% of meteorite falls, include three groups,called H, L, and LL chondrites. Carbonacous chondrites, which are generally richer in carbon andmatrix, are divided into CI, CM, CO, CV,
8、 CK, CR, CH, and CB groups. Enstatite chondritescontain virtually Fe-free enstatite and are divided into the EH and EL groups. Nearly allchondrites were metamorphosed or aqueously altered in their parent bodies to a degree that isindicated by their “petrologic type.” Type 3, or “unequilibrated,” cho
9、ndrites are the most pristine,type 6 are the most metamorphosed, and type 1 are the most aqueously altered.三种最主要的球粒陨石是三种最主要的球粒陨石是:碳质球粒陨石,普通球粒陨石,顽辉石球粒陨石。这些陨石被分为13个组。普通球粒陨石普通球粒陨石占所有陨石的约85%,分为三组:H, L, LL。碳质球粒陨石碳质球粒陨石富含碳和基质,它分为:CI, CM, CO, CV, CK, CR, CH, CB。顽火辉石球粒陨石顽火辉石球粒陨石含无铁的顽火辉石,被分为EH和EL两组。从球粒陨石的岩石学
10、类型来说,几乎所有的陨石在其母体的时候都经历了不同程度的水改变和热变质作用。Type3型(非平衡型)球粒陨石是最原始的,而type6是热变质最深的,type1是经历水蚀变最严重的。Asteroids that melted supply us with three major classes of meteorites: irons, which are FeNisamples from the cores of asteroids; metal-free silicate rocks from asteroidal mantles and crusts,which are called ac
11、hondrites as they lack chondrules; and stony irons, which are impact-generatedmixtures of achondrites and FeNi metal (Fig. 2). Most irons (85%) are divided into thirteengroups (called IAB, IIAB, IIC, etc.), which come from separate parent bodies. The remaining15% probably come from another 50-odd pa
12、rent bodies. There are two major types of stony irons:2pallasites, which are olivinemetal mixtures from at least 5 different bodies, and mesosiderites,which are mixtures of metal and basaltic material.小行星的熔融分异形成的小行星的熔融分异形成的3类陨石是类陨石是铁陨石铁陨石,来自小行星的Fe-Ni核心;石陨石石陨石,来自小行星的壳幔,因为缺少球粒又称无球粒陨石;石铁陨石石铁陨石,非球粒陨石和Fe
13、-Ni金属的混合物(图2)。85%的铁陨石被分为13个组(AB, AB, C等等),他们来自于母体的碎片;其余15%可能来自50-odd母体。石铁陨石主要有两类:橄榄陨铁(至少来自5个不同母体的橄榄石和铁的混合物)和中铁陨铁(铁和玄武岩物质的混合)。The four largest groups of achondrites are the howarditeeucritediogenite group (abbreviatedHED), aubrites, ureilites, and angrites. Eucrites are basalts and gabbros that have
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